The present invention relates to surveillance systems for determining the directions of electromagnetic-radiation sources and the frequencies at which they radiate. It is particularly beneficial for surveillance systems that employ antenna arrays having irregular shapes.
Arrays of antenna elements are sometimes used for general electromagnetic surveillance; it is desirable for the equipment to monitor several directions and frequencies simultaneously. To monitor a particular direction, the signals from the antenna elements are subjected to different amounts of phase change so that the signals from all of the elements add in phase if the source of the signals is located in the direction being monitored. To change the direction being monitored, one must change the phase shifts applied to the various signals.
One way to monitor several directions simultaneously is simply to send the element signals in parallel to different groups of phase shifters that favor different directions. It has been found in the past, however, that it is not necessary to use such a large amount of hardware to monitor several directions simultaneously. For example, many schemes have employed Fourier transformation or circular-convolution techniques to perform much or all of the signal processing for multiple directions at the same time with the same circuitry. However, most of these techniques rely on a favorable antenna-element geometry. That is, they rely on the geometric properties of linear, circular, or other arrays having regular, simple geometric shapes. Such schemes cannot be used if the array platform does not lend itself to such a simple array geometry.
An object of the present invention is to monitor several directions and frequencies simultaneously in such a way that much of the signal processing for all of the signals is performed by common equipment without requiring that the array have a simple geometry.